This invention relates to fluidized bed type reactors used for the combustion of crushed coal in industrial or utility type applications.
The combustion of crushed coal in a fluidized bed is dependent upon the temperature of the bed, maintaining sufficient oxygen for combustion, and the retention time, i.e., holding the particles of fuel in the bed for a time sufficient to attain a temperature that effects combustion. The retention time for fuel particles in the bed is chiefly a function of their size, the large particles remaining in the bed to effect combustion while small particles called "fines" are quickly ejected therefrom as partially burned "char. " The exact boundary between "fines" and the larger particles is a function of the superficial velocity in the bed. If the small particles of coal and char could be agglomerated or fused into larger particles and again injected into the combustion bed, their residence time or retention there would necessarily increase and the efficiency of combustion would also increase accordingly.
It is known that a mixture of coal fines and char may be compressed into larger particles or chunks that when burned in a fluidized bed will be retained therein for a time sufficient to effect complete combustion. A specific binding agent such as bentonite is frequently added directly to the fines before they are compressed, whereby the compacted mass resulting therefrom will be held in the fluidized bed until combustion is complete.
This type of operation may prove to be highly effective, but it does require additional material at an additional cost, and it frequently requires further steps of material preparation and waste disposal.
It has been found that certain types of medium and highly volatile bituminous (agglomerating) coals go through a plastic stage during a heating process to thereby reduce the surface area and reactivity of the coal and also increase its tendency toward agglomeration. The present invention accordingly requires that unburned carbon and char leaving a fluidized bed be collected downstream from the bed and mixed with crushed agglomerating type coal before it is compressed into large chunks and directed back into the fluid bed to thereby increase its retention time during the combustion process.